As the Internet environment becomes popular, networks are developed from original cable networks to wireless networks. In recent years, wireless networks advance rapidly and wireless network services become increasingly popular, and wireless network applications start attracting people's attention, and one of the wireless network applications is a localization service.
The localization function provides useful information to mobile device users, and we can get related information of our neighborhood through the location information. For commercial applications, a transportation of goods can be managed more effectively and flexibly, and thus more and more companies adopt a localization system to improve their service and operation efficiency. At present, a global positioning system (GPS) is a common localization system, which provides convenient localization services as well as related applications of a mobile device. However, the present existing GPS not only comes with a high price, but also has technical difficulties of receiving satellite signals and using the GPS indoors or using it in a city with tall buildings all over the city. Unlike high power GPS localization systems, a low power indoor localization system starts blooming.
Short-distance wireless transmission technologies such as Bluetooth (IEEE 802.15.3), Wireless Fidelity (Wi-Fi, IEEE 802.11) and ZigBee (IEEE 802.15.4) used in a small area are developed and used for our living environment continuously.
A location-bound wireless localization system needs not to receive satellite signals above the Earth's surface, but uses a receiver located at a target to receive a signal transmitted by each beacon transmitter within a tracking area and its identification code (ID) to estimate the location of the target. A general location-bound wireless localization method such as radio frequency triangulation or radio frequency fingerprinting adopts a receive signal strength indicator (RSSI) as a basis for positioning a target.
The radio frequency triangulation uses the intensity of a received RSSI to estimate the distance between the receiver and the beacon transmitter and receives at least three signals transmitted from a beacon transmitter to perform localization of a target.
The radio frequency fingerprinting does not use the intensity of the RSSI for the estimation directly, but it compares a model created by the RSSI before positioning the target. Such comparison model must go through an actual site calibration to establish an individual model according to a RF characteristic value of each the beacon transmitter obtained in different environments, and the RSSI values at different indoor environments are received. Since the actual partition and construction material of buildings vary, and the scattering or attenuation so produced is different in each different environment, therefore it is necessary to input a planar map of the tracking area into a localization system, and physically move the receiver to every corner of the tracking area in order to save all RSSI values and establish a model of the environment before the localization system adopting the aforementioned method can be built. Actual positions established by the RSSI values and received by a receiver that is installed onto the target, the identification code of the beacon transmitter, and the model and map of the environment are used for performing an accurate localization.
From the description above, the location accuracy of the location-bound wireless localization system depends on the accuracy and the stability of the received RSSI values. Since these short-distance wireless transmission technologies are operated at a frequency of 2.40˜2.49 GHz, the interference occurrence rate becomes increasingly higher as the frequency of using other wireless transmission devices in the same channel increases. Once if any interference occurs, the quality of transmitting signals via wireless transmissions drops significantly, and the RSSI values become distorted and unstable accordingly.
For the localization system, high efficiency and high accuracy are required, and thus we need a method to effectively overcome the problem of signal interference occurred during the localization process.